% % % % % % % % %% %% %% %% %% %% %% % GEOLOGIC HISTORY % %% %% % % Legend %% %% %% %% %% %% %% % % % % % % HOLOCENE: %% % Pl-mv Pka %%% Sediments Mt Margaret U. Kerichwa Tuffs % % % % %% %% % Longonot (0.2 - 400 ka): trachyte stratovolcano and associated deposits. Materials exposed in this map % %% %% %% %% %% %% % section are comprised of the Longonot Ash Member (3.3 ka) and Lower Trachyte (5.6-3.3 ka). The % Pka' % % % % % % L. Kerichwa Tuff % % % % % % Alluvial fan Pleistocene: Calabrian % % % % % % % Geo% lo% gy of the Nairobi Region, Kenya % trachyte lavas were related to cone building, and the airfall tuffs were produced by summit crater formation % % % % % % % % % % % % % % % % % Pna % % % % %% % (Clarke et al. 1990). % % % % % % Pl-tb % % Narok Agglomerate % % % % % Kedong Lake Sediments Tepesi Basalt % % % % % % % % % % % % % % % % %% % % % 37.0 °E % % % % 36.5 °E % % % % For area to North see: Geology of the Kijabe Area, KGS Report 67 %% % % % Pnt %% % PLEISTOCENE: % % %% % % % Pl-kl %% % % Nairobi Trachyte % %% % -1.0 ° % % % % -1.0 ° Lacustrine Sediments % % % % % % % % Pleistocene: Gelasian % % % % % Kedong Valley Tuff (20-40 ka): trachytic ignimbrites and associated fall deposits created by caldera % 0 % 1800 % % ? % % % 0 0 % % % 0 % % % % % 0 % 0 8 % % % % % 4 % 4 Pkt % formation at Longonot. There are at least 5 ignimbrite units, each with a red-brown weathered top. In 1 % % % % 2 % 2 % % Kiambu Trachyte % Pl-lv % % % % % % % % % % %% % % Limuru Pantellerite % % % % some regions the pyroclastic glass and pumice has been replaced by calcite (Clarke et al. 1990). % % % % Volcanics % % % % % % 1 P-Kta-s % % % % % % 8 % % % % % 0 % % % % % 1 Pnp % % 0 % % % % 6 % % % % 2 0 Nairobi Phonolite % % % % % 4 0 Pl-lt %% % % % % % 0 % Limuru Trachyte % % Barajai Trachyte: (0.37-0.41 Ma) five aphyric trachyte flows distinguished from the Plateau Trachytes by % % % 0 % % % Holocene % % % % % % % % % %% % % 1 Pl-kva % % % 6 Baker et. al (1988) based on element ratios; otherwise they are indistinguishable. May be earliest eruptive % % % 00 % % % % % % % % % % % % % % Pl-tt % % % % Miocene % % % % 2000 Pl-ltv Tigoni Trachyte % products from Suswa. Mapped areas are tentative and based on written description of extent in Baker et. Lt-a % %% % % % % % Longonot lower trachyte % % % % % %% % % % % % % % % %% %% % % % % % % %% %% % % % % % % % % % 2 % Kedong %% % al (1988). % % 4 ! % % Mev % % Pl-ket % 0 0 % % % Pl-kat % % % Esayeti % 0 0 % % % % Pl-lta Karura Trachyte % % % % 4 1 % % % % % Pl-kva 8 Longonot ashes % % % % DDDDD % 2 0 % % % % %% % 0 % % % % % Githunguri % % % % % % % % % % % % %% % % % Mt. Margaret: welded and unwelded trachyte tuffs erupted from a small cone that rises 120m above the rift % ! %% % % % % DDMDknDp D %% % % % % %% % % % % % % % % % % Pl-ket % % % Kandizi Phonolite % % % % % % % % % % % Kabete Trachyte % % % valley floor (Clarke et al. 1990). % % % % DDDDD %% % % % %% % %% % % % %%%% %%%%%% Pleistocene: Upper DDDDD %%% %%%% Mva %%% %%%% 160 Pl-ngv ! %% %%% 0 Athi Tuffs Ol Tepesi Basalts (1.4-1.65 Ma): alkaline basalts with sparse plagioclase and olivine phenocrysts (Baker %%% %%%% 0 Ngong Hills %%% %% 0 Pl-kvt % %% 2 Kedong Valley Tuff DDDDD %% 0 % % 2 et al. 1977), distinguished by Baker and Mitchell (1976). % 0 % %%% %% % 2 % Mmt % % 0 DDDDD % 2 % % % 0 Mbagathi Trachyte % % % % % % 0 % 2 % % % % % 0 % % % % 2 Pliocene % % 0 % DDDDD % % % % % 2 % %%% 2 Pleistocene: Ionian Limuru Trachyte (1.94 - 2.64 Ma): contains characteristically clustered groups of K-feldspar phenocrysts, 2 % % %% % % %% 18 DDDMkDp D % %% % 0 0 % % % 0 P-Kta-s % % % 0 % %% % Kapiti Phonolite % tends to form bouldery outcrops, and grades upwards into pantellerite. These were erupted as a series of % % P-Kta-s % %% % DDDDD % % % Kinangop Tuff soils % % % S1 % %% % % % % % %% % Suswa shield trachytes % conformable flows with reverse polarity, that overtopped the escarpment in this area. A thickness of 400m % % DDDDD %%%% % %%% %%% is exposed in the eastern rift escarpments (Baker et al. 1988). %%%% %%% Limuru P-Kta Metamorphics %%%% %%% ! Pl-bt Kinangop Tuff %%%% %%% Barajai Trachyte, %%%% %%% % %% %% % Pka % % % % X Tigoni Trachyte: very fine grained trachyte exposed in rivers and where it has been quarried for building %% % % % 2 %% % 00 Undifferentiated % % 0 0 % % %% % 0 % % stone. Originally named the “Karura Quartz Trachyte”, Saggerson (1991) renamed the formation to avoid %% % % % 8 %%%%%%%%%%%%%%%%% ! % % % % City Road-major %% 1 faults-large confusion with the Karura Trachyte described below. %%% 18 %%% 00 %%% A' ! %%%% Town Road-minor % % % faults-small % %% %% %% %% % Karura Trachyte: fine grained grey trachyte between the Tigoni and Kabete trachytes. Similar to the % % A % % Pl-lt % % ! % Ruiru ! % Pl-lt % %% % % ! Village Road-track % Nairobi Trachyte but is stratigraphically higher and weathers to a spotted appearance (Saggerson 1991). % rivers % % 1800 %% %% % % % Pl-tt %%%% %%%% 200m-contour rail Kabete/Ruiru Dam Trachyte: grey-green porphyritic trachyte that reaches 30m thick in boreholes. The %%%% %%% Kiambu % % % % Ruiru Dam and Kabete trachytes are indistinguishable, though the Ruiru Dam trachyte may be thicker %% % % ! %% %% %% %% % (approx. 60m). Saggerson (1991) suggested these trachytes were equivalent in age. % % % STRUCTURE %% %% %% %% %% %% % % The metamorphic rocks have been subjected to several stages of deformation (descriptions in Warden & % % % % % Ngong Hills (2.53-2.58 Ma): remnants of an old volcanic cone which had an estimated original diameter of % Pkt Horkel 1984), but the recent rift volcanics are relatively undeformed. Many low magnitude tremors have been % % % % % % 11km prior to being cut by the rift boundary faults. Composed of basanite, tephrite, and some nephelinite, % recorded, but the January 6, 1928 quake (Ms 6.9) near Lake Bogoria indicates the modern potential for large % % % Pnt % % % some lavas are noted by Saggerson (1991) to contain megascopic fragments of gneiss. Oldest associated % Pl-kat earthquakes associated with the rift boundary faults (Zielke & Strecker 2009). % % % % Mva % % 180 lavas are likely Miocene in age (see Kandizi Phonolite). % 0 % % % % % % % PALEONTOLOGY % % % % h % PLIOCENE t % Bones and plant remains were found in the sediments of the Kandizi river valley, as well as the teeth/tusks of u %%%% G %% F % . Kinangop & Kerichwa Valley Tuffs (3.34-3.70 Ma): trachytic tuffs that are often welded, and overlie the %%% o Hippopotamus gorgops. Handaxes have also been reported from the area around the Gicheru diatomite beds. % r A % Pl-lt %% % a y %% Pl-ket Nairobi Trachyte. Ages are from the Kinangop Tuff, with which the Kerichwa tuffs have been correlated % Mva r Chert flakes and tools have also been found in Nairobi National Park (Saggerson 1991). b % e %%% Pl-kl a e %%% % (Baker et al. 1988). Ages presented by Saggerson (1991) are considered too old due to presence of n %%% %%% t % o o %%%%%% % d % 2 E % feldspars that formed before tuff eruption. Bleaching and clay alteration are common, likely representing %%% 0 ECONOMIC DEPOSITS n %%% 0 a o % % 0 s i % Kikuyu % 0 % t s % 0 weathering prior to the eruption of the Limuru Trachytes (Saggerson 1991). % 0 r ! s Many of the volcanic rocks are quarried as building stones, especially the Kerichwa Valley Tuff which provides %% 2 % 0 0 Pka' e e % % 0 2 % 0 2 00 v % 0 %% 0 e %% 2 l % : %% 0 the "Nairobi Stone". Diatomite from the Munyu wa Gicheru lake deposits, 1.65-1.96 Ma (not shown, Trauth et % a % %% G t %% i %%% % e Narok Agglomerate: light brown agglomerate with numerous lithic clasts, including blocks of trachyte and g % %% i %% % al. 2007) and ferricrete are also excavated in this region. %% o l D % Mva o % . % phonolite. These occupy the same horizon as the Kerichiwa Valley Tuffs (Matheson 1966). %% g 7 % y % 2 0 9 % % 0 0 % o t % 6 0 r % f WATER RESOURCES 2 1 1 0 o % t % 0 %% 8 h p % % 0 % 0 0 % e Nairobi Trachyte (3.17-3.45 Ma): greenish-grey trachyte, occasionally with tabular feldspar phenocrysts. A e % % 0 The rivers draining the Kikuyu highlands are perennial, and a good aquifer exists between the Kerichwa Valley % 0 0 %% % N % R % 8 %% % o % 1 % % % number of thin flows reach a cumulative thickness of 90m. At a quarry in Nairobi this trachyte is separated S % %% % Tuffs and the underlying phonolite. However, the fluoride content tends to be high (Saggerson 1991). r % %% % % %% % t G % %% % h % % % Nairobi % %% % K % %% % from the Nairobi Phonolite by a thin tuff (Saggerson 1991). M % %% % % %% , % % %% % ! % % a %% a % %% % % % % c e % %% % r % % % GEOTHERMAL PHENOMENA % h %% % A % a % % 0 % % 0 % % 6 k a % Kiambu Trachyte: light grey trachyte with numerous feldspar phenocrysts. In some regions lava is % % % 1 Elevated carbon dioxide soil gas and an active fumarole (89°C), are associated with Mt. Margaret. Silicified o % w % s % s % % % - u % vesicular with green chalcedony amygdules. Likely a single flow of limited extent, maximum thickness is % pyroclastic material suggests more vigorous geothermal activity in the past (Clarke et al. 1990). % % % T % 0 S % % 0 % 6 % % 1 h % % e % % 0 46m, and it is overlain by the Nairobi Trachyte (Saggerson 1991). 0 i % % 8 1 k h %% % t % a % % % % f % % % % A o %% % % % r % % e y % % % % % a g % Nairobi Phonolite (5.20 Ma): black to blue phonolite erupted as a number of flows. Upper flow sections are % % % % , o % % l % % % % 0 K % o % % % 0 G % % 8 vesicular, but amygdules are rare. Can be distinguished from the Kapiti phonolites by the lack of large e % % 1 1